The present invention relates to a control apparatus of a four-wheel drive vehicle which has an engine of which an operating mode is switchable between an all-cylinder operating mode and a reduced-cylinder operating mode, and to the four-wheel drive vehicle.
Conventionally, four-wheel drive vehicles are known to comprise a power unit provided with a transfer case for driving rear wheels. The power unit includes an engine, a transmission, and a front-wheel differential device, is mounted on a front part of a vehicle body, and drives left and right front-wheels (main drive wheels) of the four-wheel drive vehicle. Further, a propeller shaft extending in front-and-rear directions of the vehicle body is coupled to the transfer case, and a rear-wheel differential device is coupled to a rear end part of the propeller shaft, so that left and right rear-wheels (auxiliary drive wheels) of the four-wheel drive vehicle can also be driven.
In the four-wheel drive vehicle, a coupling capable of varying a transmission torque may be disposed on the propeller shaft. When the coupling is fully fastened (i.e., engaged), an output torque of the engine is evenly applied to the front and rear wheels (four-wheel drive state), when the coupling is fully released (i.e., disengaged), the output torque of the engine is only applied to the front wheels (two-wheel drive state), and when the coupling is between the fully engaged state and the fully disengaged state, the distribution of the transmission torque applied to the rear wheels is adjusted according to the engaged state.
Further, as the transfer case, a pair of bevel gears meshed with each other, specifically, a bevel gear provided on an extended line of a central axis of the front-wheel differential device and a bevel gear provided on an extended line of a central axis of the propeller shaft, are used to transmit power to the propeller shaft extending in the front-and-rear directions of the vehicle body from the front-wheel differential device, of which a central axis extends in a width direction of the vehicle body.
With regards to four-wheel drive vehicles, in the four-wheel drive state where the front and rear wheels are driven, the distribution of the output torque of the engine to the rear wheels is increased compared to the two-wheel drive state where only the front wheels are driven, leading to drive loss and degradation in fuel economy; therefore, four-wheel drive vehicles normally travel in the two-wheel drive state, travelling in the four-wheel drive state only when necessary.
However, since the output torque of the engine varies due to intermittently performed combustion inside a combustion chamber of the engine, the variation in output torque is subsequently applied to the transfer case via the transmission and the front-wheel differential device, so that in the two-wheel drive state, a drivetrain from the bevel gear of the transfer case to the rear wheels, i.e., including the propeller shaft and the rear-wheel differential device, rotates in a non-power transmission state where power is not transmitted from the drivetrain.
Therefore, depending on the oscillation frequency of the torque variation of the engine, the drivetrain having a predetermined characteristic frequency of vibration with respect to torsional vibration co-resonates with the frequency of the torque variation of the engine and causes a large vibration in the drivetrain. Due to this vibration, noise is generated by teeth rattling between the pair of bevel gears, for example, which may cause noise inside a cabin of the vehicle.
FIG. 18 is a chart illustrating relationships between an oscillation frequency of torque variation of an engine and a torque transmission characteristic with respect to torsional vibration of a drivetrain, in a four-wheel drive vehicle and a two-wheel drive vehicle having conventional basic structures, respectively. A waveform W1 of the transmission characteristic (the waveform indicated by the solid line) with respect to the torsional vibration of the four-wheel drive vehicle and a waveform W2 of the transmission characteristic (the waveform indicated by the dashed line) with respect to the torsional vibration of the two-wheel drive vehicle, which are illustrated in FIG. 18, both have resonance peaks P1 and P2 at frequencies below a practical range of the engine (below a frequency fL). Within the practical range of the engine (the frequency fL and above), the waveform W1 of the four-wheel drive vehicle has a resonance peak P3 which is not seen in the waveform W2 of the two-wheel drive vehicle. Consequently, noise may be generated in the four-wheel drive vehicle due to vibration at the peak P3.
With regards to the resonance peak P3 of the drivetrain peculiar to the four-wheel drive vehicle, noise generation due to the resonance of the drivetrain and the resulting teeth rattling between the pair of bevel gears, for example, may be suppressed by increasing the torque distribution from the two-wheel drive state, where only the front wheels are driven, to the rear wheels via the coupling, so as to apply a load on the drivetrain to increase the torque to be applied to the rear wheels within an engine operating range in which the drivetrain resonates with the torque variation of the engine.
JP2001-277881A discloses an art for increasing a distribution of an output torque of an engine to rear wheels within a knocking range of the engine operation, so as to suppress vibration caused by knocking from being transmitted from a transfer case to a rear-wheel differential device and noise generation. However, it will be appreciated that JP2001-277881A does not disclose suppressing teeth rattling between gears of a drivetrain, which includes a propeller shaft and a rear-wheel differential device, from a bevel gear of a transfer case to rear wheels, due to resonance caused by torque variation of an engine in a four wheel drive vehicle.
Meanwhile, among four-wheel drive vehicles of a type which switches its driving state between a two-wheel drive state and a four-wheel drive state, four-wheel drive vehicles are known which include an engine having a plurality of cylinders and of which an operating mode is switchable between an all-cylinder operating mode, where all of the plurality of cylinders are operated and a reduced-cylinder operating mode, where one or some of the plurality of cylinders are operated, so as to improve a fuel economy of the engine.
In such a four-wheel drive vehicle, since a frequency of torque variation of the engine is different between the all-cylinder and reduced-cylinder operating modes of the engine even at the same engine speed, engine speeds at which a drivetrain from a bevel gear of a transfer case to rear wheels, which includes a propeller shaft and a rear-wheel differential device, resonate with the torque variation of the engine are also different therebetween.
For example, when the engine includes four cylinders, in a reduced-cylinder operating mode where operation of two of the cylinders is suspended, a frequency of torque variation of this engine is half of that in an all-cylinder operating mode at the same engine speed, and an engine speed at which the drivetrain resonates with the torque variation of the engine becomes twice the engine speed at which the drivetrain resonates in the all-cylinder operating mode.
FIG. 19 is a chart illustrating relationships between an engine speed and a varying torque of a drivetrain, in an all-cylinder operating mode and a reduced-cylinder operating mode of a four-wheel drive vehicle. As illustrated in FIG. 19, in the four-wheel drive vehicle including a four-cylinder engine, a waveform W11 of the varying torque of the drivetrain in the all-cylinder operating mode where the four cylinders are operated (the waveform indicated by the solid line) has resonance peaks P11 and P12 at engine speeds below a practical range of the engine (below an engine speed NL), and has a resonance peak P13 at an engine speed within the practical range of the engine (the engine speed NL and higher). Further, a waveform W12 of the varying torque of the drivetrain in the reduced-cylinder operating mode where two of the cylinders are operated (the waveform indicated by the dashed line) has resonance peaks P21, P22, and P23 at twice the engine speeds of the peaks P11, P12, and P13, respectively.
Therefore, even if noise generation is suppressed in the all-cylinder operating mode by increasing the torque distributed via the coupling to the rear wheels within the operating range where the drivetrain resonates with the torque variation of the engine in the all-cylinder operating mode, noise may still occur within the operating range where the drivetrain resonates with the torque variation of the engine in the reduced-cylinder operating mode.
Additionally, the increase of the torque distribution to the rear wheels via the coupling to suppress noise generation in the drivetrain due to the torque variation of the engine may lead to an increased drive loss due to mechanical losses from the drivetrain, and may result in degradation of fuel economy.